1. Field of the Invention
This invention relates to a semiconductor laser device having a visible or near-infrared light semiconductor laser element to be used as a source of light for a bar-cord-reading device, a pointer, a pick-up of a photo disc device, an optical measuring apparatus and etc..
2. Description of the Related Art
As an example of a near-infrared light semiconductor laser element, one having a so-called inner stripe structure is known (for example, Jpn. Pat. Appln. KOKAI Publication No. 57-159084).
This semiconductor laser element is constructed as shown in FIG. 1 in such a manner that an electric current barrier layer 102 consisting of n-GaAs is first formed on the upper surface of a p-GaAs substrate 101, a stripe groove 103 having a stripe-like opening is formed in the electric current barrier 102 consisting of n-GaAs, and then a plurality of layers viz. a first clad layer 104 consisting of p-Ga.sub.0.7 Al.sub.0.3 As, an active layer 105 consisting of p-Ga.sub.0.95 Al.sub.0.05 As, a second clad layer 106 consisting of p-Ga.sub.0.7 Al.sub.0.3 As and a cap layer 107 consisting of n-GaAs are stacked in this order over the n-GaAs electric current barrier 102. Additionally, an n-side electrode 108 is formed on the surface of the cap layer 107, while a p-side electrode 109 is formed on the back surface of the p-GaAs substrate 101.
In the near-infrared light semiconductor laser element constructed in this manner, an electron introduced from the anode 109 moves from the p-GaAs substrate 101 to the n-side electrode 108, passing through the stripe groove 103 formed in the electric current barrier 102, the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104, an n-Ga.sub.0.95 Al.sub.0.05 As active layer 105, a p-Ga.sub.0.7 Al.sub.0.3 As second clad layer 106 and an n-GaAs cap layer 107.
In this case, an n-p junction formed between the n-GaAs electric current barrier 102 and the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 is reverse-biased, whereas a p-n junction formed between the p-GaAs substrate 101 and the n-GaAs electric current barrier 102, as well as a p-n junction formed between the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 and the n-Ga.sub.0.95 Al.sub.0.05 As active layer 105 are forward-biased.
Namely, when electric current is passed into the device through the n-side electrode 108 and the p-side electrode 109, only the limited region formed in the stripe groove 103 becomes an electric channel.
Accordingly, the laser oscillation is initiated only at the region near the p-Ga.sub.0.95 Al.sub.0.05 As active layer 105 which corresponds to this current channel.
FIGS. 2 and 3 respectively illustrate an energy band at the portion which transverses the n-GaAs electric current barrier 102.
For example, the electric potentials, at the time of passing current, of each of the p-GaAs substrate 101, the n-GaAs electric current barrier 102, and the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 become such as shown in FIG. 2.
In this FIG. 2, .DELTA.E.sub.c denotes an amount of discontinuity of conductive band at the hetero junction of the laser element; whereas .DELTA.E.sub.v denotes an amount of discontinuity of valent electron band at the hetero junction.
In view of the relationships between potentials, each of the p-GaAs substrate 101, the n-GaAs electric current barrier 102, and the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 can be regarded as being an emitter (E), a base (B) and a collector (C) respectively.
Accordingly, it will be easily understood that as long as majority carriers (electrons) are not injected into the n-GaAs electric current barrier 102 corresponding to the base, the current passing through the electric current barrier 102 may be very little which can be practically neglected.
Thus, some of minority carriers (positive holes) to be injected into the base from the emitter by the forward bias reach to the collector to become a drift current, the remaining minority carriers are recombined with the majority carriers.
Accordingly, the base is charged positively as shown in FIG. 3, to a potential prevents injection of the minority carrier (positive hole) from the emitter into the base.
As a result, unless the electric charge is eliminated by injecting the majority carrier into the base, the electric current larger than this level can no more be possible to be supplied.
When the electric current barrier 102 functions in this manner, the portion which can be excited by the electric current is only the n-Ga.sub.0.95 Al.sub.0.05 As active layer 105 disposed right above the stripe groove 103.
The light h.nu. thus generated at this portion is guided by slab-like (laminar) light waveguide channel formed by the p-Ga.sub.0.95 Al.sub.0.05 As active layer 105 and a pair of clad layers, i.e. the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 and a p-Ga.sub.0.7 Al.sub.0.3 As second clad layer 106.
At this occasion, part of the guided light leaks from the relatively thinner portions of the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer 104 disposed on both sides of the stripe groove 103, and is absorbed by the n-GaAs electric current barrier 102 as shown by the arrow in FIG. 1.
With the function of this light absorption, the guided light is locked in the waveguide channel right above the stripe groove 103 so that the mode status is considered as being stabilized.
By the way, as the light is absorbed in this manner, an electron-positive hole pair is generated within the n-GaAs electric current barrier 102.
For example, the positive hole indicated by a white circle in FIG. 3 is discharged by the potential gradient into the p-Ga.sub.0.7 Al.sub.0.3 As first clad layer (collector) 104. As a result, only the electron indicated by a black circle is left in the n-GaAs electric current barrier (base) 102. This phenomenon can be considered as being an injection of electron into the base.
If this injection of electron is represented by the value of electric current (optically induced current) I.sub.OPT, the electric charge can be eliminated when the value of I.sub.OPT is larger than the value of I.sub.REC, which is obtained by converting the magnitude of recombination (i.e. a cause of above-mentioned charge) into an value of electric current. As a result, the n-GaAs electric current barrier 102 can no more keep its function.
Accordingly, it can be said that the relationships of I.sub.OPT &lt;I.sub.REC is an essential condition for the inner stripe structure to function its current-blocking property.
This condition can be met when the diffusion length Lp of minority carrier (positive hole) is smaller than the thickness d.sub.CB of the n-GaAs electric current barrier 102, i.e. when L.sub.p &lt;d.sub.CB is realized.
FIG. 4 illustrates the relationship between the diffusion length of minority carrier and density of majority carrier.
According to this FIG. 4, when the electron density is in the prescribed range of from n=2 to 3.times.10.sup.18 cm.sup.-3 to 2 to 3.times.10.sup.-9 cm.sup.-3 that would be easily realized, the value of the diffusion length Lp of minority carrier (positive hole) becomes 1 .mu.m or less, so that it is possible to make the thickness d.sub.CB of the n-GaAs electric current barrier 102 to about 1 .mu.m or less.
A semiconductor laser element having these features is realizable and satisfactory when these features are considered from the viewpoint of the technical level of manufacturing semiconductor such as crystal growth, as well as the viewpoint of the dimensional limitation to be imposed as an optical element of a semiconductor laser element.
However, even with this semiconductor laser element, there are following problems, if the n-GaAs electric current barrier 102 is made into p-type.
Namely, in this case, the diffusion length Ln of electron, i.e. minority carrier would be several times as large as that of the diffusion length Lp of minority carrier (positive hole) when the n-GaAs current barrier 102 is made into p-type.
It follows that it is difficult to manufacture the semiconductor laser element, for the reason mentioned above.
The first feature of the conventional semiconductor laser element resides in that the electric current barrier is made into n-conductive type, and the second feature thereof resides in that the thickness of the electric current barrier is made such as to correspond to the diffusion length of the minority carrier.
The conventional semiconductor laser elements are characterized in two points. First, the current barrier is of n-conductivity type. Second, the current barrier has a thickness determined based on the minority-carrier diffusion length.
GaAs and AlGaAs have been exemplified as two alternative materials of the-conventional elements. This is simply because only these two materials were known at the time of inventing the conventional semiconductor laser element. To,day, some other materials are available, which can be applied to the conventional element.
As a matter of fact, a structure having these two features are employed in a red visible semiconductor laser element using a material of (Ga.sub.x Al.sub.1=-x).sub.0.5 In.sub.0.5 P (0.ltoreq..times..ltoreq.1), which is developed later on and now is mass-produced.
FIG. 5 illustrates a schematic structure of a red visible semiconductor laser element adapting a structure containing these two features.
This semiconductor laser element is of so-called SBR (Selective Buried Ridge) structure, which is disclosed for example in Jpn. Pat. Appln. KOKAI Publication No. 62-200785, or Jpn. Pat. Appln. KOKAI Publication No. 62-200786. In this semiconductor laser element, a double hetero junction structure comprising an n-GaAs buffer layer 112, an n-InGaP buffer layer 113, an n-InGaAlP clad layer 114, an InGaP active layer 115 and p-InGaAlP clad layers 116, 117 and 118 is formed on an n-GaAs substrate 111.
The p-InGaAlP clad layer 117 is made of an Al-poor composition, and functions as an etching-stopper layer, whereas the p-InGaAlP clad layer 118 constitutes a stripe-like rib.
A p-InGaAlP contact layer 119 and a p-GaAs contact layer 120 are formed over this p-InGaAlP clad layer 118. Further, an n-GaAs current barrier layer 121 is formed on the side walls of these p-InGaAlP clad layer 118, p-InGaAlP contact layer 119 and p-GaAs contact layer 120.
Moreover, a p-GaAs cap layer 122 is formed over the p-GaAs contact layer 120 and n-GaAs current barrier layer 121. A p-side electrode 123 is formed over the p-GaAs cap layer 122, while an n-side electrode 124 is formed on the back surface of the n-GaAs substrate 111.
In this visible red semiconductor laser element constructed in this manner, an electron introduced from the p-side electrode 123 moves from the p-GaAs cap layer 122 to the n-side electrode 124, passing through the p-GaAs cap layer 122, the opening porting of the n-GaAs current barrier layer 121, the p-GaAs contact layer 120, the p-InGaAlP contact layer 119, the p-InGaAlP clad layers 118,117 and 116, the InGaP active layer 115, the n-InGaAlP clad layer 114, the n-InGaP buffer layer 113, the n-GaAs buffer layer 112, and the n-GaAs substrate 111.
In this case, the structure differs from that shown in FIG. 1, with respect to the crystal material constituting the layers, and also with respect to the stacking manner such that the structure as a whole is reversed in this case, so that the direction in flow of the electron is reversed. However the feature of confining the passage of electric current and light only to the striped region is the same.
In the semiconductor laser having such an inner stripe structure as mentioned above (to be more specific, an inner stripe structure that can be classified as being a refraction index guided type), it is possible to carry out both of the functions, i.e. current blocking and light absorption in a single layer so that the stabilization of lateral mode in posture of light can be relatively easily realized.
Further, since this structure is suited for use in the case where a large output is required, it has been applied to many of a near-infrared semiconductor laser element and a visible semiconductor laser element.
FIG. 6 schematically illustrates the structure of a semiconductor laser element having an inner stripe structure, which can be classified to a gain guided type.
Since this structure is suited for use in the case where a large output is required, it has been applied to many of a near-infrared semiconductor laser element and a visible semiconductor laser element.
In this semiconductor laser element, an n-GaAs buffer layer 132, an n-InGaP buffer layer 133, an n-InGaAlP clad layer 134, an InGaP active layer 135, a p-InGaAlP clad layers 136, an n-GaAs barrier layer 137 are formed on an n-GaAs substrate 131. In this structure, an opening 150 reaching to the p-InGaAlP clad layers 136 is formed in the n-GaAs barrier layer 137, and then a p-GaAs cap layer 138 is formed thereover.
Further, a p-side electrode 139 is formed on the upper surface of the p-GaAs cap layer 138, and an n-side electrode 140 is formed on the back side of the n-GaAs substrate 131.
Thus, this semiconductor laser element corresponds to the one having SBR structure shown in FIG. 5 wherein the thickness of p-InGaAlP clad layer 118 is uniformly formed. Although, the semiconductor laser element constructed in this manner is larger in astigmatic aberration, and inferior in optical property as compared with the semiconductor laser element having SBR structure, a large number of the semiconductor laser element constructed in this manner is still produced now, since the manufacturing process is relatively easy.
In the case of this structure, although the n-GaAs barrier layer 137 is not imposed to function to absorb light, since the absorption more or less of laser beam in the semiconductor laser element of ordinary design can not be disregarded, an n-type semiconductor crystal material is employed for the n-GaAs barrier layer 137.
As explained above, in the conventional semiconductor laser element of inner stripe structure as shown in FIGS. 5 and 6, the barrier layer is confined to the n-type in conductivity, thereby restricting the polarity of electric source for actuating the semiconductor element.
The semiconductor laser element is generally provided at the back side of the substrate with a heat sink in order to effectively remove the heat generated at the active layer thereof.
Generally, the heat sink is kept in same potential as that of the housing of the element, and connected to a common terminal. Accordingly, a positive electric source is exclusively employed in the near-infrared semiconductor laser element shown in FIG. 5, while a negative electric source is exclusively employed in the red visible semiconductor laser element shown in FIG. 6. Such a restriction to the polarity of the electric source is often inconvenient in the practical use.
Further, even if n-type conductivity is used for the electric barrier layer, an invalid current passing through the electric barrier layer, i.e. an electric current which is consumed for the light contributing in no way to the laser output, may often not to be disregarded.
As explained above, there is a defect in the conventional semiconductor laser device in that since the electric barrier layer is confined to an n-type conductivity, the polarity of the electric source is also required to be restricted.
Further, there is a problem in the conventional semiconductor laser device in that the invalid current can not be disregarded even if an n-type conductivity is employed for the electric barrier layer.